DE69405191T2 - SELF-LIMITING MICROWAVE SECTOR - Google Patents

Abstract

A self-limiting dispersion of a conductive material in a binder is disclosed wherein the conductive material consists essentially of a carbon material having a structure selected to undergo percolation when dispersed in the binder at a concentration of from about 10 weight percent to about 45 weight percent. The binder comprises a thermoplastic material selected to go through a first order phase transition or a second order transition at a temperature from about 300 DEG F. to about 480 DEG F. and the dispersion is formulated near the conductance percolation threshold so that its electrical conductivity drops precipitously (e.g., at least about 1.5 orders of magnitude) above the binder transition temperature. The dispersion of the conductive material in the binder is especially suitable for use as a self-limiting microwave heater material which is either a molding composition, or surface coating composition, and especially a composition that can be applied to a substrate by printing.

Description

Area of Expertise

The present invention is subject to a wide range of applications, but is particularly suitable for use in connection with self-limiting microwave heaters. The present application describes compositions and methods for making such heaters. The present invention will be particularly described in these contexts.

Underlying state of the art

Cooking food and heating substances using microwave radiation has become increasingly popular and important in recent years due to its speed, economy and low energy consumption. However, microwave heating of food products has disadvantages. One of the main disadvantages is the inability to brown or sear the food product to make it similar in taste and appearance to a traditionally cooked preparation.

Various methods have been attempted in the prior art to overcome the problem of browning. One such method for browning foods and other materials involves the use of a metal-deposited coating on paperboard. The prior art method for producing this coated paperboard required several steps.

First, metal particles are vacuum-evaporated onto a film, preferably an oriented polyester film. The film is then laminated onto the paper. The laminate of metal-evaporated film/paper usually has to be applied to a specific part of the food packaging, which requires a relatively complicated window processing operation. The window processing operation requires that the metal-coated laminate is cut before it is processed. The window processing can also produce only rectangular-shaped laminates.

In addition to the complexity of the prior art process, there are several other disadvantages. With vacuum deposition, it is difficult, if not impractical, to develop a particular pattern or shape on the applied coating that would be useful for controlling the heating of the food product. It is also difficult to change the formulation of the coating or the thickness of the coating at specific locations on the film to meet different heating requirements. This is particularly significant when different foods are heated together in a microwave oven.

In many microwave applications, additional heat transfer is required to crisp up the surface of the preparation, e.g. pizza crust, waffles, french fries and the like, or a faster heating rate and higher temperatures are required for acceptable product quality, such as for popcorn. While the additional heat input is essential for these applications, it is simultaneously desirable that this microwave-interacting material or "susceptor" should have the ability to limit its upper temperature range in order to avoid the potential to scorch (or burn) the support material onto which the microwave-interacting material is applied. such as paper, cardboard packaging material, thermoplastic film and the like. The potential for scorching or burning of the preparation itself which is placed on the carrier material is also a problem.

Tighe et al., U.S. Patent No. 4,876,423, describe a method for controlling the heating behavior in a microwave interactive coating by combining certain selected metal particles and conductive particles. Tighe et al. note that a mixture of aluminum particles and carbon black or a mixture of aluminum particles and conductive particles such as titanium carbide or zinc oxide have been found to improve control over the degree of heating. Tighe et al. used heat-resistant thermoplastic resins as the materials of choice for the binder to protect the metal particles and conductive particles from overheating and theorized that as the glass transition temperature (Tg) of the resin is reached, the binder expands so that at some point metal particle-to-metal particle contact is lost, preventing further heating until the binder cools and contracts, bringing the particles back into contact. Tighe et al., column 6, lines 40-62.

In contrast, Tighe et al., U.S. Patent No. 4,959,516, illustrate in Example 13 that carbon black alone in combination with a polyethersulfone resin-type binder did not serve as a coating of a self-limiting microwave susceptor because high temperatures were reached very quickly and the dangers of thermal burnout became apparent, e.g., smoke and fire. In contrast, when aluminum flakes were used in the coating, a non-metallic object (a ceramic plate) heated to 254.44ºC (490ºF) within four minutes and remained at a high level at 260ºC (500ºF)

Harrison, U.S. Patent No. 4,917,848 describes a process for making a microwave-interactive sheet material for use in microwave cooking by providing a receptor layer and applying to the layer a composition consisting of a liquid component containing a slurry of microwave-interactive particles no larger than 50 to distribute the composition over the surface of the receptor layer. This is followed by drying the composition to keep the particles distributed and to fix the particles in such distribution to ensure that they form a layer receptive to microwave radiation.

These coatings are an example of a microwave-interacting coating that is not self-limiting. To prevent excessive heating, the weight of the coating or the content of interacting particles must be controlled so that the composition simply equilibrates with its environment.

Stone, U.S. Patent No. 4,866,232 describes a food package for heating or cooking a food product inside the package by microwave heating. Microwave responsive spots are applied to the food packages by a printing process, the ink used in this printing process comprising metal particles dispersed in an ink-like substance.

Tighe, U.S. Patent No. 4,864,089 discloses a composition for obtaining confined heating by microwave radiation which comprises a resin binder with conductive and semiconductive particles coated on a support. Selecting the amount of materials used for the composition of the coating controls the efficiency of the conversion and allows for repeated use without burnout.

This reference also notes that the coating mass affects the amount of heating and that increased coating thicknesses allow higher temperatures. Coating thicknesses of 19 microns allow heating to 260ºC and 13 microns to reach temperatures of 165ºC. The use of thermoplastic resins for the binder protects the pigments from overheating based on the assumption that when the glass transition temperature is reached, the binder expands enough to remove the pigment-to-pigment bond, preventing further heating until the binder cools and shrinks, whereupon the pigment particles again contact each other. The patentee describes low pigment loadings with thermosetting resins.

Ochocki WO 90/04516 describes a coated laminate of a microwave permeable support layer with a printed susceptor lamina consisting of a cured dispersion of an organic ink carrier having two types of dispersed particles stirred therein. One consists of an electrically conductive, microwave-interacting carbon particle and the other consists of an electrically non-conductive, stirred particle with properties of a mineral damper for conduction, diffusion and dispersion. of the energy captured by the conductive particles. Of the two particles stirred, only the carbon particle interacts directly with the microwave energy.

Disclosure of the invention

Accordingly, the present invention relates to a novel self-limiting dispersion for producing a self-limiting microwave heater which substantially overcomes one or more of the limitations and disadvantages of the related art. Such a dispersion is defined in claim 1.

The invention is based on the discovery of a self-limiting dispersion of a conductive material in a binder, the conductive material being a carbon material whose structure is selected to undergo percolation when dispersed in the binder at a concentration of from about 10% to about 45% by weight. It has been discovered that the binder of use in this connection comprises a thermoplastic material selected to undergo a first order phase transition or a second order phase transition at a temperature of from about 148.8°C (300°F) to about 248.88°C (480°F), particularly when exposed to microwave radiation. Above these temperatures, the dispersion is below the percolation limit of conductivity.

The dispersion maintains substantially the self-limiting temperature when irradiated with microwave radiation of about 4 to about 16 watts/cm or more. In another aspect, the invention employs conductive carbon black or graphite as the conductive material, wherein the binder is a polycarbonate, a methylpentene copolymer, polyvinyl alcohol or a cellulose ester.

The dispersion also creates a microwave-responsive coating that can be printed onto a substrate. This coating overcomes the problems inherent in vacuum-deposited metal coatings because the coatings can be printed exactly where they are needed. In addition, coating patterns, coating formulations, and coating thicknesses can be varied using conventional printing processes. A printing process also allows the use of materials other than vacuum-deposited metals as microwave-responsive materials, as well as providing the opportunity for a wide range of heating temperatures and a wide variety of applications.

Compositions that meet the desired parameters can also be extruded as a film and used to form the entire packaging piece or used as a patch applied to other packaging. The extruded compositions can be thermally formed into plates or cups that would interact with microwaves and limit the temperature. The compositions can also be molded using conventional molding techniques such as injection molding.

The invention further relates to an article of manufacture as defined in claim 11, comprising a self-limiting microwave heater.

Finally, the invention relates to a method for heating food as defined in claim 27.

Preferred embodiments are defined in detail in the dependent claims.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

The accompanying drawings are intended to provide a further understanding of the invention and illustrate various embodiments of the invention and together with the written description serve to explain some of the principles of the invention.

Short description of the drawings

Figs. 1, 2, 6-9 and 12-14 are graphs plotting the change in resistivity versus temperature in a conventional oven for a conductive carbon black material together with a resin binder at various pigment:binder ratios.

Fig. 3-5 and 10, 11 are plots of power fraction versus time and temperature versus time at different absorbed powers of a self-limiting microwave heater irradiated by a Gerling microwave device.

The cheapest way to carry out the invention

Microwave responsive materials (MRMs) can convert microwave energy into heat. This is accomplished by the electrical conductivity of the microwave responsive material. The materials that exhibit these properties are hereinafter referred to as conductive material(s), microwave responsive material(s) or material(s).

The microwave responsive materials within the scope of this invention include any material having suitable conductivity properties such that a thin layer of the material can convert significant microwave radiation into thermal energy. Moreover, the microwave responsive material may have various properties depending on the formulation, the type of binder used, or the particle size and shape of the microwave responsive material. The properties of the substrate onto which the material is laminated, printed, or coated, such as orientation, heat cure temperature, and melting point or glass transition temperature (where the substrate is a thermoplastic material), as well as the adhesion between the coating and the substrate, will affect the response of the materials to microwave energy.

The type and amount of microwave responsive materials used in the coating composition generally determines the extent of interaction with the microwaves and hence the extent of heating. In a preferred embodiment, where the material used is conductive as defined herein, the extent of The heat generated is a function of the surface conductivity, which is the product of the volume conductivity of the material and the thickness of the material.

Since one aspect of the present invention involves self-limiting microwave heaters, not only must the type and amount of microwave responsive materials be considered, but also the type and amount of polymeric materials into which the microwave responsive materials are incorporated, i.e., the polymer backbone. The design of coatings, films or extruded materials having the necessary properties to obtain the self-limiting microwave heaters of the present invention takes into account the fact that conductive microwave heaters operate due to their electrical conductivity, which is attributable to the microwave responsive material incorporated in the polymer backbone. Most polymers are moderately good electrical insulators, generally having bulk conductivities below about 10⁻¹²/ohm-cm, whereas a microwave-responsive material such as graphite has a bulk conductivity in the plane of the stacked aromatic ring structure inherent in graphite of about 10⁵/ohm-cm and about 10-10²/ohm-cm normal to the plane. Soot particles, which are aciniform or grape-shaped clusters of submicron-sized graphite-like spheroids, are also fairly good conductors.

As can be understood from the foregoing, there is a difference in bulk conductivity between graphite or carbon black and most polymeric materials. A blend of microwave responsive materials and polymer would therefore result in a material with intermediate conductivity, and in general the conductivity of a blend will of a good conductor to a poor conductor may be a function of their relative concentrations. A small number of unconnected, microwave-responsive particles of high conductivity in a polymer backbone would have very little effect on conductivity. An electric field applied to the mixture remains concentrated in the regions between the particles, and little current flows between them. At higher particle concentrations, however, the particles begin to touch and fuse into connected structures. At this point, charge carriers can migrate between the electrodes without leaving the conductive phase, and the electric field cannot condense in the polymer backbone. The conductor volume concentration (i.e., the concentration of microwave-responsive particles) at which an idealized unbounded conductive structure forms is called the percolation limit. Well below the percolation limit, the mixture of particles and polymer backbone is an insulator; above percolation, it is a conductor; and near percolation, conductivity is extremely concentration dependent. For randomly distributed spherical carbon particles, percolation would occur at a concentration of about 30 wt.%.

However, the actual percolation limit for a carbon black composite is quite dependent on the carbon type. For Ketjenblack, an aciniform or highly aggregated linear porous particle structure of carbon black, the percolation is around 3 wt%, while for split carbon black (a lightly aggregated solid structure) it is over 20 wt%. This phenomenon could be explained by the non-random distribution of the non-spherical soot particles and the fact that carbon particles with high aspect ratios combine at lower concentrations.

The concentration of conductive materials such as carbon at which percolation occurs is an important consideration in the design of self-limiting heaters and particularly self-limiting microwave heaters. Although percolation will occur with some carbon black materials as low as about 3%, the conductivity (i.e., the inverse of the resistivity) at the percolation limit is quite low compared to carbon materials which percolate at about 20 wt.% or about 30 wt.%. The latter type of carbon black materials with higher concentrations are preferred for the self-limiting heaters of the present invention and particularly for the self-limiting microwave heaters of the present invention since higher conductivities upon percolation will allow a manageable microwave heating element to achieve high microwave absorption ratios at a reasonable thickness. Microwave heating is a phenomenon based on surface conductivity, i.e. up to a surface conductivity of about 0.005/ohm, better conductive materials generate more heat when exposed to microwave radiation.

However, this is only part of the explanation. The other phenomenon that must be taken into account is the semi-crystalline structure of a conventional polymer binder or backbone as well as thermal transformations of the polymer. These thermal transformations include a first order phase transformation, which is the glass transition of the polymer. A second order phase transformation occurs at the melting temperature of the polymer. In the first order phase transformation or the second order phase transformation, the polymer will undergo an increase in volume or show thermal expansion.

Particles of particulate microwave-responsive material tend to cluster in the amorphous sections of semicrystalline polymers, which form continuous pathways around the crystallites, even though the crystallinity can be quite high. Since the particles are excluded from the crystalline sections, they can percolate in the pathways at a low overall concentration. Accordingly, the percolation limits of microwave-responsive materials in combination with polymers are quite dependent on the respective properties.

In addition to percolation, quantum mechanical tunneling plays an important role in the standard description of conduction from microwave-responsive material to polymer. Even above percolation, the particles do not quite touch each other. They are separated by thin polymer layers that create potential barriers for the charge carriers (i.e., electrons or vacancies). The carriers do not have enough energy to overcome the barriers and they cannot jump from particle to particle. Since the barriers are very thin, significant charge transfer can nevertheless occur through the tunneling mechanism. It is therefore assumed that composites of particles made of microwave-responsive material in a polymer framework comprise an interconnected network of particles separated by very thin polymer layers.

At room temperature and above, the current is extremely dependent on the separation of the particles due to tunneling between the particles. Temperature fluctuations, which would cause relatively small changes in the separation of the particles, can therefore have disproportionate influences on the electrical conductivity. The polymer framework generally has a higher coefficient of thermal expansion than the particle, and increasing temperature usually wedges the islands apart. The current due to tunneling is very sensitive to insulator thickness, and large changes in conductivity can result from small polymer expansions. In addition, when the particle concentration is near percolation, where conductance is highly responsive to structural changes, reductions in conductance with temperature can be particularly dramatic. Near the thermal transformation of the polymer binder, where the coefficient of thermal expansion is large, very large thermal conductance coefficients are seen in mixtures close to the percolation limit.

Both first and second order transformations of a polymeric binder can cause orders of magnitude conductivity changes in a composite near percolation, and both are used here for microwave temperature control. Semi-crystalline polymers whose first order transformation temperature (Tg) is below room temperature can be controlled by a second order transformation temperature that is above room temperature, i.e. the melting temperature (Tm). When the temperature of a composite with a semi-crystalline binder undergoes a second order transformation, the crystallite regions "melt" at Tm, i.e. they transform from the crystalline phase to an amorphous phase. During the transformation, the thermal expansion coefficient undergoes a discontinuity. If the conductive phase is near percolation, this can cause a fairly large drop in conductivity over a narrow temperature range. For thermostable microwave operation, an ideal binder would expand volumetrically would be constant below Tm, but would show a large expansion throughout the melting range. High temperature binders made from amorphous polymers can also be used for microwave temperature control. This time the glass transition temperature is above room temperature, and the jump in the thermal expansion coefficient at Tg provides temperature control.

At the transition temperature of the polymer backbone, the backbone expands and pushes the microwave-responsive particles apart, and, aggravated by accompanying changes in the phenomena of tunneling and percolation, the conductivity drops sharply. Below the transition temperature, the composite is a conductor, while above the melting point it is an insulator. The increase in resistivity with temperature above the transition temperature is called the PTC (positive temperature coefficient of resistivity) effect. However, there are cases where a decrease in resistivity with temperature above the melting point occurs, called the NTC (negative temperature coefficient of resistivity) effect, which should be minimized or avoided in a composition for self-limiting heaters. Consequently, the drop in conductivity associated with melting in the crystalline regions can be followed by an increase in conductivity at higher temperatures, i.e. NTC phenomenon. It has been theorized using a soot-polyolefin model that attractions between polar groups on the soot particles trigger the formation of a soot network in the less viscous polymer framework around the crystal nuclei, thereby increasing conductivity.

Consequently, it is evident that the selection of polymers as well as microwave responsive materials is crucial to obtain self-limiting microwave heaters.

The design of the heater must also take into account the fact that the impedances of the composite of particles of microwave-responsive material and polymer have significant capacitive components. The spacing between the particles is close enough and the adjacent regions are large enough that significant amounts of charge can be stored at the interfaces. The composite can be considered analogous to a resistor connected in series with a parallel resistor-capacitor combination, rather than a simple resistor. At high frequencies the capacitive elements can carry a significant portion of the current, while at low frequencies they will not. This specific capacitance depends on the separation of the particles from each other in the polymer backbone, but this is a linear relationship. Since the PTC effect arises from the great sensitivity of the current due to tunneling to particle separation, it will therefore be understood that at higher frequencies a greater proportion of the current will flow through the capacitor-like structure of the composite compared to lower frequencies, and accordingly the PTC effect will be less prominent at higher frequencies. This effect must be taken into account in the selection of microwave responsive materials and polymer composites particularly for microwave heaters operating at frequencies from about 2.0 GHz to about 3.0 GHz and especially at about 2.45 GHz plus or minus 50 MHz.

Although the theory was put forward at one point is that the microwave responsive material that would perform best in the self-limiting microwave heater of the present invention would be that which provides the greatest conductivity at low concentrations, it has been determined in accordance with the present invention that this is not necessarily the case. Microwave responsive materials which provide high conductivities in composites at low concentrations will also percolate at lower concentrations. The conductivity of these composites near percolation, where temperature control is useful, may be relatively low. Thus, to optimize conductivity upon percolation, microwave responsive materials with less structure are superior, even if they provide less conductivity at low concentrations. The materials must be selected so that they are not overly agglomerated, but rather have sufficient conductivity and lack of structure so that percolation occurs at a conductivity high enough to equip the self-limiting microwave heater to develop temperatures suitable not only for heating, but also in some applications for browning and roasting food preparations.

The present invention is therefore based on the discovery of certain types of microwave responsive conductive materials and polymer backbone materials in combination, selected according to the foregoing indications, and used in the form of a thin film, coating, printed material, molded material or extruded material to provide a self-limiting microwave heater for heating and particularly for browning or roasting foodstuffs.

The conductive materials which have been found to be particularly suitable in this regard are those which consist essentially of conductive carbon black materials and conductive graphite materials. Mixtures of different types of conductive carbon black materials may be used, as may mixtures of different types of graphite materials. In addition, mixtures of conductive carbon black materials with conductive graphite materials may also be used in accordance with the present invention.

The preferred conductive material consists essentially of the conductive carbon blacks. The conductive carbon black materials in this context include those having a surface area, as measured by nitrogen absorption, of less than about 160 m²/g, and more particularly from about 5 to about 160 m²/g, and more particularly from about 15 to about 150 m²/g. The conductive carbon black materials are also selected to have a particular particle structure, as measured by dibutyl phthalate (DBP) absorption, of less than about 150 cc/100 g, and more particularly from about 30 to about 150 cc/100 g, and preferably from about 50 to about 140 cc/100 g. The conductive carbon black materials of the present invention should also have an average particle size of from about 10 to about 120 nm, and more particularly from about 15 to about 100 nm, and preferably from about 17 to about 90 nm. Conductive carbon black materials that fall within the foregoing ranges of nitrogen surface and DBP absorption can also be used as blends. Thus, conductive carbon black materials having a particle size of about 20 nm to about 35 nm can be used together with carbon blacks having particle sizes of about 40 to about 90 nm. The ratio of the larger particle size carbon blacks to the smaller particle size carbon blacks can be anywhere from about 80/20 wt.% to about 60/40 wt.% Carbon blacks for these purposes can be obtained from Huber Engineered Carbons and are listed below in Table 1 and Cabot Special Carbon Blacks in Table 2. TABLE 1 HUBER ENGINEERED CARBON S

* Arosperse series, treated at higher temperatures to reduce volatile content. TABLE 2 CABOT SPECIAL SOLAR

The "ARO" series of materials in Table 1 are modified grades of the "N" grade materials which have been treated at higher temperatures to reduce the volatile content.

The various polymers that have proven suitable in the previous contexts consist essentially of polycarbonates, methylpentene copolymer, polyvinyl alcohol, polyvinylidene fluoride and cellulose esters such as cellulose acetate/propionate, cellulose butyrate and cellulose acetate/butyrate.

The polycarbonate resins which are representative of resins which control Tg include those polymers derived from the reaction of a dihydrogenated aromatic alcohol with phosgene, the dihydrogenated aromatic compounds including bisphenol A, bisphenol F and the like. These resins are described in more detail in Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Ed. under the heading "polycarbonates", which is incorporated herein by reference.

TABLE 1

The menthylpentene copolymer, which is characteristic of hearts that control by means of Tm, is obtained by dimerization of propylene to a monomer, 4-methylpent-1-ene, which is polymerized to form the methylpentene copolymer.

The polycarbonate resins are commercially available products that can be purchased from General Electric Plastics as Lexan PPC 4504 and PPC 4704 (polyphthalate carbonate resins) with FDA-compliant food grade for Lexan PPC 4501 and PPC 4701, respectively. Other suitable polycarbonate resins include Calibre polycarbonates from Dow Chemical Company and APEC,

High temperature polycarbonate and Makrolont polycarbonate supplied by Miles Polymer Division. Particularly suitable polycarbonates are of the grade APEC HT DP 9-9330, 9340, 9350, 9360 and 9370.

In a preferred aspect of this embodiment, when the microwave responsive material consists essentially of carbon black or graphite, the microwave responsive material combined with binder will preferably have a resistivity in the range of from about 25 ohms per square to about 100,000 ohms per square, and more preferably from about 100 ohms per square to about 50,000 ohms per square at room temperature.

In a preferred embodiment of the invention, the microwave responsive material will be suitable for food packaging. As an alternative, the microwave responsive material will be separated from the food by a film or other protective means.

It is preferred that the dispersion exhibit rapid heating to a desired temperature with subsequent leveling off of temperature without arcing while exposed to microwave radiation. The temperature at which the dispersion limits is hereinafter referred to as the self-limiting temperature. Generally, the dispersion will operate at a temperature in the range of about 148.8°C (300°F) to about 480°F; and preferably about 148.88°C (300°F) to about 232.22°C (450°F) or about 176.66°C (350°F) to about 204.44°C (400°F).

The microwave-responsive material is mixed with a binder to form a composition for extrusion, compression molding or coating. Binders used in this invention may comprise an extrusion or compression molding composition of polycarbonate, polyvinylidene fluoride or a methylpentene copolymer, or any aqueous dispersion (e.g., latex) or dispersion in an organic liquid thereof or solution thereof, which may be used in a roll coating, spray coating, dipping or printing process or a solder part molding process, the latter to form an unbonded film from the dispersion. The binder must have good thermal resistance and suffer little or no degradation at the temperatures generated by the microwave responsive material. It should also have adhesive properties which would allow it to be bonded to a substrate.

The binder and microwave responsive material are generally mixed in a suitable ratio so that the microwave responsive material in the form of a thin film from extrusion, molding or coated or printed can convert microwave radiation into heat and raise the temperature of a food preparation placed thereon, while still having sufficient binder to be extruded or molded and, when coated or printed, to adhere to the carrier or film. There should also be sufficient binder to prevent arcing in the microwave responsive material. The ratios are also dependent on the percolation and extent of thermal expansion of the binder.

In general, the ratio of microwave responsive material to binder will depend on the microwave responsive material and binder selected due to the solids content. In a preferred In an embodiment where the microwave responsive material is carbon black or graphite and the binder is polycarbonate, polyvinylidene fluoride, methylpentene copolymer, polyvinyl alcohol, or cellulose ester, the ratio of microwave responsive material to binder is, by weight, from about 10:90 to about 45:55 or more, and more preferably from about 15:85 to about 40:60.

The microwave responsive material, thus in combination with the binder, comprises the dispersion of the present invention, and the binder will undergo a first order phase transition or a second order phase transition at a temperature of about 148.88°C (300°F) to about 248.88°C (480°F). The dispersion is near the percolation limit at about room temperature (approximately 21.11°C (70°F)) and its conductivity drops above the phase transition temperature of the binder. The drop in conductivity is at least about 1.5 to about 2 orders of magnitude or more compared to the pre-phase transition conductivity of the binder, and in some cases at least about 5 to about 7 orders of magnitude or more, again compared to the pre-phase transition conductivity of the binder. Consequently, the range for the drop in specific conductivity is from at least about 1.5 orders of magnitude to about 7 orders of magnitude or more.

Other materials may be incorporated into the composition, such as surfactants, dispersants, lubricants and other conventional additives for compositions for extrusion, molding, coating or printing.

The dispersion is advantageously applied as a film, which from about 0.00254 mm (0.1 mils) to about 0.254 mm (10 mils) thick, such film obtained by the processes described herein including coating, casting, printing or extrusion. Films from about 0.0762 (0.3) to about 0.01524 mm (0.6 mils) are particularly useful for heating foods such as popcorn and films from about 0.01524 (0.6) to about 0.0254 mm (1 mil) are particularly useful for heating pizza.

The coating may be applied using conventional roll, dip or spray coating techniques or printing techniques such as rotogravure, flexography and lithography. After the coating composition has been applied, it may be dried using conventional ovens normally available in a coating or printing process.

The composition for extrusion and compression molding are used in conventional extrusion and compression molding equipment.

In general, any amount of coating can be used in the present invention. The amount of heat generated will vary according to the amount and type of coating applied to the substrate. In a preferred embodiment, when the coating material is carbon black or graphite, the amount of coating will range from about 0.908 to about 11.35 kg per 278.7 square meters (2 to about 25 pounds per 3,000 square feet).

The coating composition can generally be coated on any support, such as paper or cardboard or any suitable film material. Usually, any support which is sensitive to microwave radiation continuous or otherwise capable of being used in a microwave process, bearing the microwave responsive coating of the present invention. In another embodiment of this invention, the coating composition is printed onto a thermoplastic film. The film may be selected from any known films such as films made of polyesters, nylons, polycarbonates and the like. The film should also have a melting point above the operating temperature of the microwave responsive material. A particularly preferred type of film includes polyester films such as Mylar (PET).

In a further embodiment of this invention, the film coated in this way is applied to a microwave-transmissive carrier. The carrier is preferably also dimensionally constant at the operating temperature of the microwave-responsive material. Other carriers such as glass, ceramic and equivalent materials can be used. Common carriers include paper and cardboard. The coating can also be applied directly to the microwave-transmissive carrier, e.g. paper or cardboard.

In another embodiment, a printing process is used to create a microwave interactive coating on paper or board, thereby providing increased flexibility in applying the coating to a substrate. Patterns can be created in the coating and they can be applied using conventional printing techniques to precisely lay the coating on the substrate. In addition, different layer thicknesses can be applied simultaneously where foods requiring different heating levels require, in the same carton or container. Printing processes require fewer steps, are more continuous processes and also avoid the problems of uniformity, outgassing and best possible control required for the metallization processes used to produce the metallized PET.

The dispersions for the extruded, molded or cast films can be used not only as patches applied to other packaging such as a paper or cardboard carrier, but can also be formed into sheets and thermoformed into plates or cups which are microwave-interactive and temperature-limiting. Thermoformed plates can be made using conventional vacuum molding processes.

The compositions of the present invention can also be formulated as injection molding compounds for use in injection molding processes well known in the art.

The following examples are illustrative.

Various coating compositions were prepared and evaluated for self-limiting microwave heater application, the results of such evaluation being given in Fig. 1-14.

example 1

Referring to Fig. 1, a polycarbonate resin manufactured by General Electric Co., Lexan PPC 4701, was dissolved in methylene chloride as a solvent to form a 15 weight percent solution. The resulting solution was mixed with carbon black obtained from the Cabot Company and designated Cabot BP160. Coating compositions comprising 30, 35 and 40 weight percent carbon black based on the solids of the formulation were prepared and coated on a paper backing to a dry coating weight of 4.086 kg/278.7 square meters (9 lb/3000 square feet). Various samples of the dried coating were placed in a conventional oven and heated to temperatures of 50, 100, 150 and 200°C. The surface resistivity (ohm/q) was measured at each of these temperatures and the results plotted as in Fig. 1. It can be seen by referring to Fig. 1 that the increase in resistivity translates into a decrease in conductivity (conductivity being the inverse of resistivity) as the temperature increases. The extent of this shift demonstrates that the coating compositions are self-limiting coatings.

Example 2

A polyvinylidene fluoride polymer (Kynar ) manufactured by ATOCHEM North America was similarly dissolved in dimethylformamide solvent to form a solution comprising 15 wt.% resin. This solution was mixed with the BP 160 carbon black at solids ratios of 83/17 and 74/26 polymer to carbon black. These coatings were applied to a paper backing at a dry coating weight of 1.362 kg/278.7 square meters (3 lb/3000 square feet). Samples of the dry coated paper were then heated in a conventional oven at temperatures of 50, 100, 150 and 200ºC and the resistivity (ohm/q) was measured at each of these temperatures. The results of these evaluations were applied as shown in Fig. 2, which again illustrates that the change in resistivity with increasing temperature means a decrease in conductivity and shows the suitability of these coatings as self-limiting microwave heaters.

Example 3

A coating of high temperature polycarbonate (Lexan PPC 4701) and carbon black (BP-160) having a 40 weight percent pigment loading prepared according to Example 1 and evaluated as shown in Figure 1 was further tested by transversely inserting the sample into an instrumented S-band waveguide operating at 2.45 GHz with a Gerling high power microwave transmitter at power levels of 139, 319, and 385 watts. Measurements of the fractional power, transmitted, reflected and absorbed, versus time, and temperature versus time were made and plotted as shown in Figures 3, 4 and 5, from which it can be seen that the sample reached a temperature of about 176.6ºC (35ºF) within about 10 seconds and maintained that temperature for a period of about 60 seconds. The percentage of microwave energy absorbed decreased as the power was increased, conclusively demonstrating that the coating composition is indeed "self-limiting".

Example 4

Microwave popcorn bags were made by laminating paper patches with adhesive to form preformed bags without heating elements, which were filled with 3.17-4.08 kg/278.7 square meters (7-9 lb./3,000 sq. ft.) of the coating of Example 3. The size and location of these spots were similar to the metallized film in the control bags, which are conventional microwave popcorn bags.

The control and test bags were filled with a standard filling of popcorn, oil and salt. These bags were heated in a Litton microwave oven at a power level of 600 watts and a frequency of 2.45 GHz for 200 seconds. The temperature response, the volume of popped kernels and the number of non-popped kernels in the test bags were slightly better than in the control bags, thus demonstrating the excellent performance of these self-limiting microwave heaters.

Example 5

A commercially available prepackaged pizza was evaluated with one of the self-limiting microwave heaters of the present invention. The pizza is placed on a paper plate with a metallized microwave responsive foil for sale. This metallized foil was replaced with paper coated with 4.086-5.44 kg/278.7 square meters (9-12 lb/3000 sq. ft.) of the coating of Example 3 and the pizza was placed on the plate with the paper coating and heated in a Litton microwave oven at 600 watts and a frequency of 2.45 GHz for 320 seconds. Reasonably satisfactory browning and toasting of the pizza crust was obtained.

Example 6a - 6f

A polycarbonate resin (12 g Lexan PPC 4701, a polyphthalate/carbonate resin from GE Plastics) is dissolved in 108 g methylene chloride. 8 g carbon black (Black

Pearls 160 from Special Blacks Division of Cabot Corp.) and 5 g of methylene chloride. This mixture (a 60/40 blend of resin and carbon black with a total solids content of 15%) was stirred until the carbon black was completely wetted. Then 175 g of steel chips were added and the mixture was stirred rapidly until the carbon black was evenly dispersed (pass at #7 on the Hageman milling gauge).

The coating was applied to paper supports using a Bird film applicator and allowed to air dry.

The resistivity versus time values were obtained by placing a one-inch wide strip of the coated paper in a forced-ventilation oven. The electrodes of a digital volt/ammeter were connected to conductive strips (Electrodag 46155 from Acheson Colloids Co.) painted across these strips at one-inch intervals and readings taken at the desired temperatures.

Using the general procedure above and adjusting the ratios as necessary, coatings were prepared with the carbon blacks shown in Table 3. In all cases, Lexan PPC 4701 was used as the binder.

The variation in resistivity with temperature for these coatings at 4.54 kg/278.75 square meters (10 lb/3000 sq. ft.) provides a good comparison of the self-limiting capabilities of these formulations. Figure 6 shows this relationship for Example 6a.

These values indicate that by using this be special polycarbonate resin with carbon blacks of the types shown in Table 3, Examples 6a - 6e, coatings were obtained which had both the specific conductivity at the desired level (hence the desired degree of microwave absorption for rapid heating) and a sufficient loss of surface conductivity (1/surface resistivity) at a desired temperature in the range of 100ºC to 250ºC to provide the essential self-limiting characteristics of these microwave interactive coatings.

Under the same conditions, the carbon blacks shown in Figures 7-9, Examples 6f - 6h (shown at 4.54 kg/278.7 sq. meters) (shown at 10 lb/3000 sq. ft.) did not provide either the desired level of specific conductivity and/or the desired amount of loss of conductivity with temperature to provide self-limiting behavior.

To confirm that these coatings do indeed provide both rapid heating and self-limiting behavior when exposed to microwaves, samples of these coatings were evaluated at 4.54 kg/278.7 square meters (10 lb/3000 sq. ft.) in the basic measurement system with the Gerling energy input. The primary components of this system are a 1.5 kW microwave transmitter operating at 2.45 GHz ± 50 MHz, a variable controller, a sample holder/transmitter, an S-band waveguide, three power meters with directional couplers, Luxtron fluoro-optic temperature measurement equipment, and a computer. This system can measure the fraction of energy reflected, the fraction of energy transmitted, and calculate the fraction of energy absorbed as a function of the time of microwave application.

The test results obtained from the Gerling system evaluating a 60/40 Lexan PPC 4701/Black Pearls 160 coating of Example 6a show that as the incident power level is increased from 139 watts to 319 watts and finally to 385 watts, Figs. 3, 4 and 5, the rate of temperature rise increases slightly and the percentage of power input decreases; however, the control temperature remains essentially constant. These results are indicative of those formulations that provide the desired rapid temperature warm-up and self-limiting control.

Figures 10 and 11 show the test results obtained by the Gerling system evaluating a 75/25 Lexan PPC 4701/Vulcan XC72 coating. At the incident power level of only 135 watts, the coating was self-limiting because the drop in absorption (which corresponds to a drop in specific conductivity) was sufficient to maintain the average temperature at approximately 176.66ºC (350ºF). However, when the incident power level was increased to 291 watts (Figure 11), the shift in power input was not sufficient to control the temperature rise, resulting in burnout and ignition of the sample. These results are indicative of those formulations that do not provide an adequate shift in specific conductivity to provide the desired self-limiting control. TABLE 3

¹ Sourced from the Special Blacks Division of Cabot Corp.

² Sourced from Huber Engineered Carbons.

Example 7

Following the general procedure outlined in Example 6, all polycarbonate resins from the series shown in Table 4 were formulated with carbon black, Black Pearls 160 from Cabot, at a 60/40 and/or 70/30 resin to carbon black ratio. The self-limiting temperature range exhibited in the Gerling system with each of the resins is shown in Table 4.

In general, the performance of all of these polycarbonate resins was quite favorable in this application. However, there were notable differences. The general purpose polycarbonates with lower Vicat softening points of about 151.66ºC (305ºF) to about 157.22ºC (315ºF) gave lower self-limiting temperatures, while the so-called high temperature grades, with higher Vicat softening points of about 160ºC (320ºF) to about 204.40ºC (400ºF) high self-limiting temperatures resulted.

The advantage is that within the polycarbonate resins alone, there is a wide range of performance characteristics that can be used to match the heater performance to the requirements of the particular packaging application. TABLE 4

¹ GE Plastics

² Plastics Group, Dow Chemical Co.

³ Polymers Div., Miles Inc.

Example 8

13.5 g of a methylpentene copolymer (DX820 from Mitsui Petrochemicals, Ltd.) was dissolved in 127.5 g of hot cyclohexane. While keeping the mixture hot, 9 g of carbon black (Black Pearls 160 from the Special Blacks Division of Cabot Corp.), 250 g of steel chips and 30 g of cyclohexane were added. Mixing was continued until the carbon black was evenly dispersed (pass number 7 on the Hageman gauge). The coating was applied to a paper backing with a Bird film applicator and immediately dried with a heat gun. The resulting coatings were approximately 2.95 kg/278.7 square meters (6.5 lb/3000 sq. ft.) of the 60/40 methylpentene copolymer/carbon black formulation.

Using similar conditions, a 65/35 formulation was also prepared and applied to paper.

The resistivity to temperature response of samples of these coatings is shown in Figure 12 (2.95 kg/278.7 square meters) (at 6.5 lb/3000 sq. ft.). Testing of samples of these coatings in the Gerling system described in Example 6 shows that the self-limiting temperature of these formulations is approximately 193.3 to 215.5ºC (380 - 420ºF).

A 70/30 blend of this methylpentene copolymer and Black Pearls 160 carbon black was prepared by forcing the blend three times through a twin-screw extruder to obtain a uniform blend. The beads from the resulting blend were pressed into 4 x 4 sheets (0.127 mm and 0.228 mm) (5 mil and 9 mil thick) using a heated platen press. The resistivity to temperature response of these sheets was quite comparable to the coatings on paper described above.

Example 9

To 9.75 g of a cellulose acetate/propionate resin (Tenite Propionate 377E from Eastman Chemical Co.) dissolved in 55.25 g of methyl ethyl ketone, 5.25 g of carbon black (Black Pearls 160 from the Special Blacks Division of Cabotcorp.), 29.75 g of methyl ethyl ketone and 150 g of steel chips were added. This mixture was rapidly stirred until the carbon black was well dispersed (about 45 min., run #7 on the Hageman gauge). This 65/35 resin to carbon black coating formulation was applied to a paper backing using the Bird film applicator as described in Example 6.

Following a similar procedure, a coating was also prepared containing a 40/60 ratio of resin to carbon black of the same materials.

The resistivity to temperature response of samples of these coatings at 4.08 kg/278.7 square meters (9 lb/3000 sq. ft.) is shown in Figure 13. These values indicate that a 65/35 formulation would be self-limiting.

Example 10

To 8 g of a polycarbonate resin (Lexan PPC 4701, a polyphthalate/carbonate resin from GE Plastics) dissolved in 72 g of methylene chloride were added 8 g of carbon black (Black Pearls 160 from the Special Blacks Division of Cabot Corp.), 30 g of methylene chloride and 175 g of steel chips. This mixture was stirred at high speed until the carbon black was evenly dispersed (about 45 minutes, run number 7 in the Hageman measuring device). To this was added 4 g of solid aluminum flakes (Stapa CVIII paste from Obron Corp., which had previously been dispersed to 40% solids in a 1:9 isopropyl alcohol/ethyl acetate mixture) and gently stirred. The resulting formulation with solids contents of 40% polycarbonate resin, 20% aluminum flakes and 40% carbon black was applied to a paper carrier using Bird film applicators.

Following a similar procedure with the same ingredients, coatings were produced that consisted of 60% polycarbonate resin, 10% aluminum flakes and 30% carbon black.

The resistivity versus temperature response of samples of these coatings at 4.54 kg/278.7 square meters (10 lb/3000 sq. ft.) is shown in Figure 14. When these values are compared to the same resin and carbon black formulations without aluminum flakes (Figure 6), they show that the aluminum flakes provide only a very small amount to the total conductivity of the coatings compared to the contributions provided by carbon black. It is also evident that when the coatings are compared at the same initial resistivity, the magnitude of the jump in conductivity with temperature is reduced in those formulations containing aluminum.

The values obtained with the Gerling test system generally confirmed the above results and showed that the aluminum flake acts primarily as a "dielectric" component rather than a "conductive" component in these coatings.

These results show that low levels of metal flakes such as aluminium can be tolerated in these coatings, but that they do not make a significant positive contribution.

Claims (29)

1. A self-limiting dispersion of a conductive material in a binder, the conductive material consisting essentially of a carbon material whose structure is selected such that a percolation limit is reached when the material is dispersed in the binder in an amount of from 10% to 45% by weight, the binder comprising a thermoplastic material selected to undergo a first order phase transition or a second order phase transition at a temperature of from about 148.8°C (300°F) to about 248.8°C (480°F), and the dispersion is near the percolation limit such that its electrical conductivity drops by at least 1.5 orders of magnitude when it is above the phase transition temperature of the binder, and the conductive material in combination with the binder has a Surface resistance which at room temperature is between 25 ohms per square and 100,000 ohms per square. 2. The dispersion of claim 1, wherein the conductive material consists essentially of carbon black or graphite having a nitrogen surface area of about 5 to about 160 m²/g, a DBP absorption number of about 30 to about 150 cc/100 µm, and an average particle diameter of about 15 to about 100 nm, and wherein the binder comprises a thermoplastic material selected to undergo a first order phase transition or a second order phase transition at a temperature of about 148.8°C (300°F) to about 248.8°C (480°F) when exposed to microwave radiation. 3. Dispersion according to claim 2, wherein the binder consists essentially of a polycarbonate, a methylpentene copoly mer, a polyvinyl alcohol or a cellulose ester. 4. The dispersion of claim 3, wherein the self-limiting temperature range is between about 148,800 (300°F) and about 232,200 (450°F). 5. The dispersion of claim 3, wherein the self-limiting temperature range is between about 176,600 (350°F) and about 204,400 (400°F). 6. Dispersion according to claim 3, wherein the conductive material consists essentially of carbon black. 7. Dispersion according to claim 6, wherein the polycarbonate consists essentially of polyphthalate carbonate. 8. The dispersion of claim 6, wherein the binder is a polycarbonate having a Vicat softening point of about 151,600 (305°F) to about 157,200 (315°F), and wherein the microwave heater has a self-limiting temperature range of about 148,800 (300°F) to about 176,600 (350°F). 9. The dispersion of claim 6, wherein the binder is a polycarbonate having a Vicat softening point of about 16,000 (320°F) to about 204,400 (400°F), and wherein the self-limiting temperature range is from about 162,700 (325°F) to about 232,200 (450°F). 10. Dispersion according to claim 3, wherein the dispersion is a surface coating mixture, a casting mixture, an extrusion mixture or a solvent cast film mixture. 11. An article of manufacture comprising a self-limiting microwave heater, the heater comprising a film of the dispersion of claim 3, the film being between about 0.00254 mm (0.1 mils) and about 0.254 mm (10 mils) thick. 12. The article of manufacture of claim 11, wherein the dispersion is a thermoplastic solid applied to a microwave transparent substrate by extrusion coating. 13. The article of manufacture of claim 11, wherein the dispersion is applied in the form of a thermoplastic film by lamination to a microwave-transparent substrate. 14. The article of manufacture of claim 11, wherein the film is applied to a microwave transparent substrate by coating or printing. 15. An article of manufacture for heating food using microwave energy, comprising a self-limiting dispersion according to claim 1 of a conductive material in a binder. 16. The article of manufacture of claim 15, wherein the conductive material consists essentially of carbon black or graphite having a nitrogen surface area of about 5 to about 160 m²/g, a DBP absorption number of about 30 to about 150 cc/100 gm, and an average particle diameter of about 15 to about 100 nn, and wherein the binder comprises a thermoplastic material selected to undergo a first order phase transition or a second order phase transition at a temperature of about 148.8ºC (300ºF) to approximately 248.8ºC (480ºF) when exposed to microwave radiation. 17. The article of manufacture of claim 16, wherein the binder consists essentially of a polycarbonate, a methylpentene copolymer, a polyvinyl alcohol, or a cellulose ester. 18. The article of manufacture of claim 17, wherein the self-limiting temperature range is between about 148.8°C (300°F) and about 232.2°C (450°F). 19. The article of manufacture of claim 17, wherein the conductive material consists essentially of carbon black. 20. The article of manufacture of claim 19, wherein the dispersion is a film having a thickness between about 0.00254 mm (0.1 mils) and about 0.254 mm (10 mils). 21. The article of manufacture of claim 20, wherein the dispersion cooperates with a microwave-transparent substrate. 22. The article of manufacture of claim 21, wherein the dispersion is a thermoplastic solid applied to a microwave transparent substrate by extrusion coating. 23. The article of manufacture of claim 21, wherein the dispersion is applied in the form of a thermoplastic film by lamination to a microwave-transparent substrate. 24. The article of manufacture of claim 21, wherein the film is applied to a microwave transparent substrate by coating or printing. 25. Article of manufacture according to one of claims 21 to 24, wherein the substrate is paper, cardboard, thermoplastic, glass or a ceramic. 26. The article of manufacture of claim 25, wherein the dispersion is applied to the substrate in varying coating thicknesses. 27. A method of heating food comprising applying microwave energy to an article of manufacture according to claim 16, wherein the food interacts with the article of manufacture, and wherein the microwave energy is sufficient to increase the temperature of the article of manufacture from about 148.8°C (300°F) to about 248.8°C (480°F). 28. A method of heating food comprising applying microwave energy to an article of manufacture according to claim 17, wherein the food interacts with the article of manufacture, and wherein the microwave energy is sufficient to increase the temperature of the article of manufacture from about 148.8°C (300°F) to about 248.8°C (480°F). 29. A method of heating food comprising applying microwave energy to an article of manufacture according to claim 19, wherein the food interacts with the article of manufacture, and wherein the microwave energy is sufficient to increase the temperature of the article of manufacture.